Four electronically low-lying states of silylene
(SiH_{2}) have been studied systematically using
high level *ab initio* electronic structure theory.
Self-consistent-field (SCF), two-configuration (TC) SCF,
complete active space (CAS) SCF, configuration interaction
with single and double excitations (CISD), and CASSCF
second-order (SO) CI levels of theory were employed with
eight distinct basis sets. The zeroth-order wave functions
of the ground (X ^{1}A_{1} or 1 ^{
1}A_{1}) and B ^{1}A_{1} (or 2
^{1}A_{1}) excited states are appropriately
described by the first and second eigenvectors of the TCSCF
secular equations. The TCSCF-CISD, CASSCF, and CASSCF-SOCI
wave functions for the B ^{1}A_{1} (or 2
^{1}A_{1}) state were obtained by following
the second root of the CISD, CASSCF, and SOCI Hamiltonian
matrices. At the highest level of theory, the CASSCF-SOCI
method with the triple zeta plus triple polarization
augmented with two sets of higher angular momentum
functions and two sets of diffuse functions basis set
[TZ3P(2f,2d)+2diff], the energy separation (T_{0})
between the ground (X ^{1}A_{1}) and first
excited (a ^{3}B_{1}) states is determined
to be 20.5 kcal/mol (0.8990 eV, 7180 cm^{-1}),
which is in excellent agreement with the experimental
T_{0} value of 21.0 kcal/mol (0.910 eV, 7340
cm^{-1}). With the same method the T_{0}
value for the A ^{1}B_{1} -- X ^{
1}A_{1} separation is predicted to be 45.1
kcal/mol (1.957 eV, 15 780 cm^{-1}), which is also
in fine agreement with the experimental value of 44.4
kcal/mol (1.925 eV, 15 530 cm^{-1}). The
T_{0} value for the B ^{1}A_{1} --
X ^{1}A_{1} separation is determined to be
79.6 kcal/mol (3.452 eV, 27 840 cm^{-1}). After
comparison of theoretical and experimental T_{0}
values for the a ^{3}B_{1} and A ^{
1}B_{1} states and previous studies, error
bars for the B ^{1}A_{1} state are
estimated to be +/- 1.5 kcal/mol (+/- 525 cm^{-1}).
The predicted geometry of the B ^{1}A_{1}
state is r_{e}(SiH) = 1.458 Angstrom and
Theta_{e} = 162.3^{o}. The physical
properties including harmonic vibrational frequencies of
the B ^{1}A_{1} state are newly determined.